Using a Mac Running OS X to Share an Internet Account

Mac OS X includes a built-in DHCP server you can use to share a single Internet connection with other devices on your local network. And if the Macintosh on which you configure the DHCP server includes an AirPort card, you can also provide a wireless AirPort network without the use of an AirPort base station.

NOTE

The function of the DHCP server is to provide and manage IP addresses to devices on the network. The DHCP server doesn't actually provide the Internet access itself; that comes from the connection method you use (such as a cable modem). The DHCP server manages the traffic between the Internet connection and the other devices on the network.

One advantage of this approach is that you don't need to add dedicated Internet sharing hardware (such as a sharing hub or an AirPort base station) to your network. A standard Ethernet hub enables you to share an Internet account over an Ethernet network, and an AirPort card enables you to share an account over a wireless AirPort network. Another advantage is that it doesn't cost anything to share an account (assuming that you already have the connection hardware, such as for an Ethernet network).

NOTE

You can use Mac OS X's built-in firewall to protect the DHCP machine from attacks from the Internet (and because it sits between the Internet and the other devices on your network, it protects those devices as well).

To learn how to configure the Mac OS X firewall, see "Defending Your Mac Against Net Hackers," p. 911.

This approach does have one significant disadvantage and one minor drawback, however. The significant disadvantage is that the Mac providing DHCP services must always be running for the machines that share its account to be capable of accessing the Internet. If the DHCP machine develops a problem, no device on the network can access the Internet. Similarly, if the machine from which the account is shared goes to sleep, the connection is lost by all the computers on the network. The less significant issue is that the DHCP services do require some processing power. These services will most likely not result in any noticeable performance decrease, but if your machine already runs at its limits, asking it to provide these services might slightly slow down other tasks.

NOTE

DHCP servers are not platform specific. For example, if you have a DHCP server running on a Macintosh, you can connect a Windows computer to the network and use the same DHCP server to share the Internet account with it. Or, you can install a DHCP server on a Windows machine and use it to share the account with Macs on the network.

Configuring a Mac to provide DHCP services to a network requires the following general steps:

Connect the Mac to the Internet.
Install the network you will use to connect the Mac with other machines.
Configure the Mac to share its Internet account.

Connecting the DHCP Mac to the Internet

It goes without saying (but I will say it anyway) that to use a Mac to share an Internet account, that Mac must be connected to the Internet. The method you use to connect to the Internet doesn't matter. You'll get the best results if you use a broadband connection, such as a cable DSL modem, but you can also share a dial-up connection if you want (don't expect speedy operation, though).

To learn how to connect a Mac to the Internet, see Chapter 10, "Connecting Your Mac to the Internet," p. 263.

Installing the DHCP Mac on a Network

The next step is to install the DHCP Mac on the network with which you are going to share the Internet connection. You can build a wired network using Ethernet, or you can install an AirPort card to connect the Mac to other AirPort-equipped Macs (you can share an account with other machines using both networking methods at the same time).

To learn how to install, configure, and use an AirPort network, see Chapter 11, "Using an AirPort Network to Connect to the Internet," p. 297.

To learn about Ethernet, see "Ethernet," p. 705.

To learn about Ethernet hubs, see "Finding and Installing an Ethernet Hub," p. 808.

To learn how to build and manage a network, see Chapter 26, "Building and Using a Network," p. 821.

Configuring the DHCP Mac to Share an Account

After you have configured the Mac for Internet access and connected it to other computers (with or without wires), you need to configure the Internet sharing services on it.

The three possibilities when you configure Internet sharing on your Mac are as follows:

Your Mac is connected to the Internet via an Ethernet connection, and it has an AirPort card installed in it.
Your Mac is connected to the Internet via Ethernet but does not have an AirPort card installed in it.
Your Mac is connected to the Internet via an AirPort base station, in which case you can share the Internet account by connecting that Mac to other computers via Ethernet to share its account via an Ethernet network (you wouldn't need to share the Internet connection with the AirPort-equipped Macs because the base station does that).

NOTE

AirPort base stations, except the original version, include an Ethernet port you can use to connect the base station to an Ethernet network. When you do this, the base station can also share a connection with machines connected to the Ethernet network. In that case, you don't need to use a Mac to share a connection.

When you configure Internet sharing on your Mac, it automatically determines which case is true for your machine and presents the appropriate options for you. To configure Internet sharing, use the following steps:

Open the System Preferences Utility and click the Sharing icon to open the Sharing pane.
Click the Internet tab. What you see depends on how the Mac is connected to the Internet. For example, in Figure 27.1, you see the Sharing Internet tab for a machine connected to the Internet via AirPort; you can tell this is so because AirPort is selected on the "Share your connection from" pop-up menu. In Figure 27.2, you see an example of a machine connected to the Internet via Ethernet. Because that machine also has an AirPort card installed in it, it can share its connection with other machines using both Built-in Ethernet and AirPort.

Figure 27.1. This Internet tab is for a computer connected to the Internet via AirPort.

Figure 27.2. The Internet tab shows a machine that is currently connected to the Internet via Ethernet, but that also has an AirPort card installed in it.
On the "Share your connection from" pop-up menu, select the Internet connection you want to share. In most cases, you will be connected by only one means, such as AirPort or Built-in Ethernet, so the choice should be obvious. However, if the Mac can connect in multiple ways, choose the option you want to share.

In the "To computers using" box are the choices you have for sharing the connection that you selected in step 3 with other computers. For example, in Figure 27.2, you can see that the Mac can share its connection with other machines over Ethernet and AirPort.
Mark the On check box for the ports over which you want to share the connection. For example, to share the Mac's connection with computers via AirPort, mark the On check box next to AirPort on the list of ports.

If you use an Ethernet port to share a connection, you might see a sheet warning you that activating sharing might cause problems for other ISP customers or violate your service agreement (some providers prohibit sharing an individual account on multiple machines, but most allow a reasonable amount of sharing such as with five or fewer computers). Read the dialog box and click OK to close it.
If you made an AirPort port active in step 4, select the AirPort port and click the AirPort Options button. Then configure the AirPort network you are creating using the resulting sheet. For example, you will name the network (if you don't want to use the default name), choose a channel and password, and so on. When you are done configuring the AirPort network, click OK.

To learn how to configure an AirPort network, see Chapter 11, "Using an AirPort Network to Connect to the Internet," p. 297.
Click Start (you can't click Start until you select at least one port on the "To computers using" list).

You might see a warning sheet that explains that activating sharing might disrupt services on the network. If you are administering the network, click Start in the sheet. If someone else administers the network on which you are sharing your connection, make sure you coordinate with that person before starting Internet Sharing.

Your connection will be shared with all the devices with which your Mac can communicate. For example, if your Mac is connected to a local network via Ethernet and you made that port active, other devices on the network can use the account via the DHCP services your Mac provides.
Quit the System Preferences Utility.
Configure the other devices on the network to use the Mac's DHCP server to access the Internet.

To learn how to install, configure, and use an AirPort network, see Chapter 11, "Using an AirPort Network to Connect to the Internet," p. 297.

To learn how to connect a Mac to the Internet, see Chapter 10, "Connecting Your Mac to the Internet," p. 263.

TIP

If you share an Internet account over AirPort, an upward pointing arrow is added to the center of the AirPort icon in the menu bar. This indicates that the connection is shared and that you can access the sharing controls from the menu.

CAUTION

If the machine sharing the connection goes to sleep, the Internet connection is lost on the network and no machine can access the Net. You should disable sleep using the Energy Saver pane of the System Preferences application when you use Internet Sharing. (In fact, when you start up sharing on a Mac that has sleep turned on, the Energy Saver button appears on the Internet pane so you can easily jump over and turn off sleep.)

If other machines with which you are sharing a connection are unable to connect to the Internet, see "The Machines with Which I Am Sharing a Connection from My Mac Can't Connect at All" in the "Troubleshooting" section at the end of this chapter.

If other machines with which you are sharing a connection lose the connection to the Internet, see "The Machines with Which I Am Sharing a Connection from My Mac Have Lost Internet Access" in the "Troubleshooting" section at the end of this chapter.